This invention relates to marine propulsion devices such as outboard motors and stern drive units and, more particularly, to water jacketed exhaust discharge systems including an exhaust gas relief arrangement.
Exhaust gas systems for internal combustion engines of outboard motors typically are exhausted downwardly through a gas expansion chamber in a drive shaft housing and then discharged into the water through a through-the-hub propeller or the like. The exhaust gas expansion chamber is jacketed with water to cool the exhaust gases and muffle sound.
At higher boat speeds, a low pressure region is created behind the gear case and propeller and exhaust gases are easily discharged into the water. At engine idle or lower boat speeds, water backs up through the hub into the exhaust gas expansion chamber and creates a static back pressure which restricts the discharge of the exhaust gases and creates rough engine operating characteristics.
Exhaust relief systems have been provided for venting the exhaust gases to atmosphere through a discharge outlet located in the drive shaft housing during engine idle and low boat speeds.
Examples of prior art constructions including an exhaust release system are disclosed in the following United States Patents:
______________________________________ Patentee Patent No. Issue Date ______________________________________ Hulsebus 3,045,423 July 24, 1962 Larsen 3,198,162 August 3, 1965 Gazzara 3,282,373 November 1, 1966 Hoiby et al. 3,296,997 January 10, 1967 Kollman 3,310,022 March 21, 1967 Boda et al. 3,350,879 November 7, 1967 Miller 3,520,270 July 14, 1970 Tado 3,577,952 May 11, 1971 Miller et al. 3,911,852 October 14, 1975 Harralson et al. 3,967,446 July 6, 1976 Maier et al. 4,036,162 July 19, 1977 Harbert 4,019,456 April 26, 1977 Harada 4,145,988 March 27, 1979 Sanmi et al. 4,303,401 December 1, 1981 Sanmi et al. 4,354,849 October 19, 1982 Nakahama 4,421,490 December 20, 1983 Hall et al. 4,507,092 March 26, 1985 ______________________________________